The move leaves the other four Executive parties - DUP, Sinn Fein, SDLP and Alliance - taking part in negotiations over the Programme for Government.

The two main parties are thought to be setting out their initial reaction to papers sent by the SDLP and Alliance in response to a document given to all five parties last Tuesday.

They are also expected to finalise a work plan, which is likely to focus on the major policy areas including health, education, the budget and dealing with the legacy of the Troubles. Two weeks had been set aside for the discussions before the Executive - now reduced from 12 to nine departments - is appointed.

The Deputy First Minister revealed he expected the D'Hondt procedure - the formula which decides the shareout of ministerial portfolios - would be run in the Assembly early next week. "What we are trying to do here is a process... which will see the maximum buy-in," Mr McGuinness said.

"If it takes the rest of this week, then it takes that, and I expect the D'Hondt mechanism will probably be run at the start of next week."

Meanwhile, senior SDLP negotiator Claire Hanna said if her party believed it could improve government by being inside it, it would not go into Opposition alongside the UUP.

She said much of the language in the DUP/SF initial document was "aspirational" and the party wanted to wait to see whether it can plug the gaps.

"We will make our decision when the job is done and we won't be dictated by the moves of other parties," she added.

After ministers are appointed the draft Programme for Government will then be put to MLAs, where DUP and Sinn Fein votes will ensure it goes through, before a period of public consultation, possibly over the summer recess.

It comes as First Minister Arlene Foster made a pitch for UUP members unhappy with leader Mike Nesbitt's move to join the ranks of the DUP. The DUP leader said she believed many rank-and-file UUP members would be disappointed by the decision, which she described as "walking off the pitch".

But she recognised the right of the UUP to take the decision to form an Opposition, which was underpinned in last November's Fresh Start Agreement between the DUP and Sinn Fein.

Mr Nesbitt, meanwhile, denied he had "jumped too soon" by announcing he would not take part in the negotiations even before they had got fully under way.

He reiterated his demand for a shake-up in the seating arrangements in the Assembly, allowing his party to sit opposite the governing parties.

"If the DUP and Sinn Féin want to send the public a clear message that things will be different this time and the days of their mutual veto are gone, there can be no stronger signal than changing the seating arrangements in the chamber," he said.

"During the election campaign Arlene Foster put great store on the symbolic importance of her being First Minister. There is equal symbolic significance in the official Opposition sitting opposite the main parties of government. That makes this a test of her credentials, her consistency and her commitment to support a truly fresh start."